Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Many cell phones and Personal Digital Assistants (PDAs), for example, incorporate microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts.
Microelectronic imagers include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid-state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect their delicate components and to provide external electrical contacts.
Many imaging devices include semiconductor dies having image sensors located on a front surface of the die to receive incoming radiation. The dies also include bond pads for electrically coupling the sensors to other circuit elements. In order to prevent the bond pads from interfering with the operation of the sensors, or limiting the size and/or location of the sensors, the bond pads can be positioned on the opposite side of the die from the sensors (e.g., on the back surface of the die). Through-wafer interconnects (TWIs) are used to conduct electrical signals from the sensors and associated internal circuitry, through the die to the bond pads at the back surface. The TWIs are typically formed by making a blind via in the die, filling the via with solder, and then grinding the back surface of the die to expose the blind end of the via, which is used to form the bond pad. A solder ball can then be attached to the bond pad and can be reflowed to couple the die to external devices.
One potential drawback associated with the foregoing approach is that, when the solder ball is later reflowed to electrically attach the die to external devices, the solder within the via may also tend to melt or at least soften. During the ensuing attach process, the solder within the via can be pulled at least partially out of the via, or can otherwise undergo deformations and/or movement that can adversely affect the electrical continuity of the TWI. In some instances, the electrical continuity of the TWI may be disrupted, causing the electrical connection between the image sensor and the external devices to fail.
Another potential drawback associated with the foregoing approach is that the number of steps required to produce the TWI increases the cost of making the associated die. For example, the use of solder in the blind via requires forming a vent hole to allow gases within the via to escape as the solder enters. Even so, the solder may include voids, which can disrupt electrical continuity and in subsequent processes, expand and create electrical shorts. Still further, the processes required to dispose the solder in the via are typically high-temperature processes, which can use up a significant portion of the thermal budget of the die. Accordingly, there is a need for an improved process for forming microfeature workpiece electrical connections, including connections between dies and external devices.